In many offshore installations, an offshore cable is connected to a hang-off interface above the surface of the water at the installation. A non-limiting example of this is a cable raised to a hang-off interface inside an offshore Monopile, such as found in wind turbines in an offshore energy park, as well as floating foundations. Such cables are often run from one Monopile/Wind Turbine foundation to another along the seabed.
There are essentially two methods for running a cable between two installations. The cable can either be connected to the first installation and then spooled out to the second installation, where the cable is cut to appropriate length and a termination device/hang-off flange installed on site to the cut end of the cable. This is a very labour intensive and costly operation, however. Alternatively, the cable may be pre-cut. In the latter situation, the cable can either be of the correct length, or more commonly either too short or too long. The drawbacks of the different situations, i.e. cable cut on site and pre-cut of the cable onshore, are described in more detail in the following, starting with the situation where the cable is cut on site.
Cutting the cable require the end of cable to be sealed off before it can be pulled into the foundation/installation thru the seawater. Then the cable ends must be equipped with cable protection which requires deck space on the cable installation vessel (designated cable lay-vessel or vessel or barge equipped with cable storage and handling equipment. Such work impacts the vessel and marine operation for many hours. A typical cable installation campaign has daily costs exceeding €100.000. In situations where the cable is too long, it is typically connected to the first installation and then spooled out to the second installation, where the cable is cut to the appropriate length and a termination device/hang-off flange installed on site to the cut end of the cable. This is a very labour intensive and costly operation, however. After the cable is pulled in and hung-off in a temporary hang-off arrangement it is common routine that the termination of the cable commences, as a separate offshore campaign offshore. This work requires multiple days of offshore work within the foundation, typical figure is three (3) days for a competent offshore termination crew adding costs of a dedicated personal transfer vessel, the offshore personal and WOW (waiting-on-weather) contingency may run above €100.000 per cable end).
The alternative, i.e. in situations where the cable is prepared on shore, it is usual to prepare a pre-cut pre-terminated cable of predetermined length on shore, and deliver the pre-cut cable to the site. The costly labour of cutting and terminating the cable on site offshore is thus avoided. In practice, however, it has proven to be very difficult to correctly estimate the correct length of such pre-cut/pre-terminated cables. The uneven surface of the seabed, unexpected geological formations and other factors very often result in the cable being either too long or too short. In the event the cable is too long, it may have to be cut and re-terminated at the site, thereby destroying the cost-savings of having pre-cut the cable on shore. Alternatively, the cable may be laid in different routing which if laid in a curve- or snaked (=multiple curves in stead of straight lay) potentially could solve the overlength issue. But this requires other measures in post-lay burial operations and adds unnecessary material costs to the cable.
In the event the cable is too short it is not fit for purpose and may, for example simply be discarded on the seabed, and a new cable must be prepared. This is obviously an extremely inconvenient and costly necessity.
The present invention provides a solution to the drawbacks identified above.